(1) Field of the Invention
The present invention relates to a printed circuit board and a method of manufacturing the same.
(2) Description of Related Art
Flexible printed circuit boards (hereinafter referred to as FPC boards) are used as transmission paths of digital signals between the central processing unit (hereinafter referred to as CPU) and a liquid crystal display in information communication devices such as mobile phones. The FPC board mainly includes an insulating layer and differential transmission paths provided on the insulating layer.
Recently, the digital signals used in information communication devices have higher frequencies due to the increase of information amount transmitted by such devices. In the FPC board, however, high frequency noise may be created by harmonics from the differential transmission path as digital signals of high frequency range are transmitted.
To shut off the high frequency noise, a shield layer made of metal is formed on the insulating layer where the differential transmission path is formed. In this case, transmission loss of digital signals in the differential transmission path is increased. Therefore, there is a limit to high frequency digital signals.
On the other hand, the information amount that the FPC board can transmit can be improved by increasing the number of differential transmission paths. In this case, the freedom of layout of differential transmission paths is limited due to high density configuration of the differential transmission paths. If widths and spacing composing the differential transmission paths is discontinuous in a longitudinal direction of the differential transmission paths, it is difficult to ensure continuity of differential impedance in the differential transmission path.
JP 2002-158411 A describes that the characteristic impedance of one area of the conductor circuit pattern is controlled by reducing the thickness of a part of the insulating layer overlapping the one area of the conductor circuit pattern compared to the thickness of another part of the insulating layer overlapping another area of the conductor circuit pattern. In this case, continuity of differential impedance can be ensured by partially controlling the differential impedance of the differential transmission path to adjust the thickness of the insulating layer. However, when the thickness of the insulating layer cannot be adjusted, it is impossible to ensure the continuity of the differential impedance. Therefore, the freedom of layout of the differential transmission path is limited.